title: Direct measurement of the Mn(II) hydration state in metal complexes and metalloproteins through 17O NMR line widths.
abstract: Here we describe a simple method to estimate the inner-sphere hydration state of the Mn(II) ion in coordination complexes and metalloproteins. The line width of bulk H2(17)O is measured in the presence and absence of Mn(II) as a function of temperature, and transverse (17)O relaxivities are calculated. It is demonstrated that the maximum (17)O relaxivity is directly proportional to the number of inner-sphere water ligands (q). Using a combination of literature data and experimental data for 12 Mn(II) complexes, we show that this method provides accurate estimates of q with an uncertainty of ±0.2 water molecules. The method can be implemented on commercial NMR spectrometers working at fields of 7 T and higher. The hydration number can be obtained for micromolar Mn(II) concentrations. We show that the technique can be extended to metalloproteins or complex:protein interactions. For example, Mn(II) binds to the multimetal binding site A on human serum albumin with two inner-sphere water ligands that undergo rapid exchange (1.06 × 10(8) s(-1) at 37 °C). The possibility of extending this technique to other metal ions such as Gd(III) is discussed.
authors.affiliation: The Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital , Harvard Medical School , 149 Thirteenth Street, Suite 2301, Charlestown, Massachusetts 02129, United States.
authors.forename: Eric M
authors.initials: EM
authors.lastname: Gale
authors.forename: Jiang
authors.initials: J
authors.lastname: Zhu
authors.forename: Peter
authors.initials: P
authors.lastname: Caravan
mesh_terms: D008345:Manganese / Q000737:chemistry*; D008667:Metalloproteins / Q000737:chemistry*; D019906:Nuclear Magnetic Resonance, Biomolecular / Q000379:methods*; D013696:Temperature; D014867:Water / Q000737:chemistry*
chemical_list: D008667:Metalloproteins; D014867:Water; D008345:Manganese